The technical field generally relates to control of NOx reduction systems to minimize the amount of slip of NH3 from the system. A common NOx reduction system includes a selective catalytic reduction (SCR) system using urea as the reductant. Certain applications that inject reductant into cool exhaust gas experience deposits in the exhaust pipe. Upon a subsequent heating operation, the deposits can clear resulting in NH3 emission spikes. When the heating is rapid and to a high temperature, for example in a soot oxidation operation for a particulate filter, the resulting NH3 emissions can be greater. Further, rapid heating on an engine exhaust system can cause the release of stored NH3 from the SCR catalyst into the exhaust stream. Certain applications include NOx determination that includes a cross-sensitivity to NH3, such that a large NH3 slip can be mis-diagnosed as a large NOx emissions event. Therefore, further technological developments are desirable in this area.